1. Field of the Invention
The present invention relates to apparatus for moving a plane mirror, rapidly and accurately, about two non-parallel axes. The apparatus of the present invention has a number of possible applications, for example, in surveying equipment or in steering arrays of mirrors for solar heating apparatus. However, the invention is especially useful for incorporation into equipment for removing port-wine stains from human beings, and will be described with especial reference to this application.
Port-wine stains are red or purple coloured birthmarks caused by enlarged blood capillaries in the dermis of the skin. Blood collects in these enlarged capillaries, causing a red or purple pigmented area which is clearly visible on the surface of the skin. Port-wine stains are a fairly common congenital defect, and if large and present on the face or neck, they can be very disfiguring. One of the most effective ways of removing port-wine stains has proved to be treatment with a laser. The laser is selected so that its wavelength is one which is preferentially absorbed by the haemoglobin of the blood, so that when the laser is directed onto the epidermis covering the port-wine stain, the haemoglobin is preferentially heated by the laser and the surrounding tissue remains comparatively unheated. The heating of the haemoglobin heats the nearby endothelial cells of the capillary wall, and the cells rupture, causing the capillary to collapse. The body's normal recuperative mechanisms repair the capillaries, or bypass the blocked capillary with new, normal size capillaries. The repaired capillaries are of decreased diameter due to scarring of the surround tissues. The scarring itself is too slight to be visible through the eipdermis, and since the new `bypass` capillaries and the repaired capillaries are of normal or near-normal diameter, the port-wine stain is greatly lightened. The treatment is repeated until the port-wine stain matches the normal skin colour. In the above-described treatment, the laser beam must be controlled very precisely, so as not to damage normal skin or over-treat any part of the port-wine stain. Hitherto, the laser beam has been directed onto the skin by a fibre-optics cable which the surgeon moves over the port-wine stain. Unfortunately there is a considerable power loss associated with the use of a fibre-optic cable, and the cable also spoils the collimation of the laser beam. A further drawback is that manually moving the end of the cable over the patient is slow and can be very time-consuming if a large port-wine stain is to be treated. It is therefore an object of the present invention to provide apparatus for directing a laser beam very accurately, but without using a fibre-optics cable.